Platelet glycoprotein VI aids in local immunity during pneumonia-derived sepsis caused by gram-negative bacteria

Experimental thrombocytopenia does not affect acute kidney injury 24 hours after renal ischemia reperfusion in mice

The pathophysiology of renal ischemia/reperfusion (I/R) injury is characterized by excessive activation of inflammation and coagulation processes followed by abnormal renal tissue repair, resulting in renal injury and function loss. Platelets are important actors in these processes, however to what extent platelets contribute to the pathophysiology of renal I/R injury still needs to be elucidated. In the current study, we treated wild-type mice with a platelet depleting antibody, which caused thrombocytopenia. We then investigated the role of platelets during the pathophysiology of renal I/R by subjecting control wild-type mice with normal platelet counts and thrombocytopenic wild-type mice to renal I/R injury. Our results showed that in the early phase of renal I/R injury, thrombocytopenia 24 hours after ischemia reperfusion does not influence renal injury, neutrophil infiltration and accumulation of inflammatory chemokines (e.g. keratinocyte chemoattractant, monocyte chemoattractant protein 1, tumor necrosis factor alpha). In the recovery and regeneration phase of I/R injury, respectively 5 and 10 days post-ischemia, thrombocytopenia did also not affect the accumulation of intra-renal neutrophils and macrophages, renal injury, and renal fibrosis. Together, these results imply that lowering platelet counts do not impact the pathogenesis of I/R injury in mice.

Clinical significance of altered collagen-receptor functioning in platelets with emphasis on glycoprotein VI

Much interest surrounds the receptors α2β1 and glycoprotein VI (GPVI) whose synchronized action mediates the attachment and activation of platelets on collagen, essential for preventing blood loss but also the most thrombogenic component of the vessel wall. Subject to density variations on platelets through natural polymorphisms, the absence of α2β1 or GPVI uniquely leads to a substantial block of hemostasis without causing major bleeding. Specific to the megakaryocyte lineage, GPVI and its signaling pathways are most promising targets for anti-thrombotic therapy. This review looks at the clinical consequences of the loss of collagen receptor function with emphasis on both the inherited and acquired loss of GPVI with brief mention of mouse models when necessary. A detailed survey of rare case reports of patients with inherited disease-causing variants of the GP6 gene is followed by an assessment of the causes and clinical consequences of acquired GPVI deficiency, a more frequent finding most often due to antibody-induced platelet GPVI shedding. Release of soluble GPVI is brought about by platelet metalloproteinases; a process induced by ligand or antibody binding to GPVI or even high shear forces. Also included is an assessment of the clinical importance of GPVI-mediated platelet interactions with fibrin and of the promise shown by the pharmacological inhibition of GPVI in a cardiovascular context. The role for GPVI in platelet function in inflammation and in the evolution and treatment of major illnesses such as rheumatoid arthritis, cancer and sepsis is also discussed.

Platelets in Sepsis: An Update on Experimental Models and Clinical Data

Beyond their important role in hemostasis, platelets play a crucial role in inflammatory diseases. This becomes apparent during sepsis, where platelet count and activation correlate with disease outcome and survival. Sepsis is caused by a dysregulated host response to infection, leading to organ dysfunction, permanent disabilities, or death. During sepsis, tissue injury results from the concomitant uncontrolled activation of the complement, coagulation, and inflammatory systems as well as platelet dysfunction. The balance between the systemic inflammatory response syndrome (SIRS) and the compensatory anti-inflammatory response (CARS) regulates sepsis outcome. Persistent thrombocytopenia is considered as an independent risk factor of mortality in sepsis, although it is still unclear whether the drop in platelet count is the cause or the consequence of sepsis severity. The role of platelets in sepsis development and progression was addressed in different experimental in vivo models, particularly in mice, that represent various aspects of human sepsis. The immunomodulatory function of platelets depends on the experimental model, time, and type of infection. Understanding the molecular mechanism of platelet regulation in inflammation could bring us one step closer to understand this important aspect of primary hemostasis which drives thrombotic as well as bleeding complications in patients with sterile and infectious inflammation. In this review, we summarize the current understanding of the contribution of platelets to sepsis severity and outcome. We highlight the differences between platelet receptors in mice and humans and discuss the potential and limitations of animal models to study platelet-related functions in sepsis.

Thromboinflammation: challenges of therapeutically targeting coagulation and other host defense mechanisms

Thrombosis with associated inflammation (thromboinflammation) occurs commonly in a broad range of human disorders. It is well recognized clinically in the context of superficial thrombophlebitis (thrombosis and inflammation of superficial veins); however, it is more dangerous when it develops in the microvasculature of injured tissues and organs. Microvascular thrombosis with associated inflammation is well recognized in the context of sepsis and ischemia-reperfusion injury; however, it also occurs in organ transplant rejection, major trauma, severe burns, the antiphospholipid syndrome, preeclampsia, sickle cell disease, and biomaterial-induced thromboinflammation. Central to thromboinflammation is the loss of the normal antithrombotic and anti-inflammatory functions of endothelial cells, leading to dysregulation of coagulation, complement, platelet activation, and leukocyte recruitment in the microvasculature. α-Thrombin plays a critical role in coordinating thrombotic and inflammatory responses and has long been considered an attractive therapeutic target to reduce thromboinflammatory complications. This review focuses on the role of basic aspects of coagulation and α-thrombin in promoting thromboinflammatory responses and discusses insights gained from clinical trials on the effects of various inhibitors of coagulation on thromboinflammatory disorders. Studies in sepsis patients have been particularly informative because, despite using anticoagulant approaches with different pharmacological profiles, which act at distinct points in the coagulation cascade, bleeding complications continue to undermine clinical benefit. Future advances may require the development of therapeutics with primary anti-inflammatory and cytoprotective properties, which have less impact on hemostasis. This may be possible with the growing recognition that components of blood coagulation and platelets have prothrombotic and proinflammatory functions independent of their hemostatic effects.

Functional significance of the platelet immune receptors GPVI and CLEC-2

Although platelets are best known for their role in hemostasis, they are also crucial in development, host defense, inflammation, and tissue repair. Many of these roles are regulated by the immune-like receptors glycoprotein VI (GPVI) and C-type lectin receptor 2 (CLEC-2), which signal through an immunoreceptor tyrosine-based activation motif (ITAM). GPVI is activated by collagen in the subendothelial matrix, by fibrin and fibrinogen in the thrombus, and by a remarkable number of other ligands. CLEC-2 is activated by the transmembrane protein podoplanin, which is found outside of the vasculature and is upregulated in development, inflammation, and cancer, but there is also evidence for additional ligands. In this Review, we discuss the physiological and pathological roles of CLEC-2 and GPVI and their potential as targets in thrombosis and thrombo-inflammatory disorders (i.e., disorders in which inflammation plays a critical role in the ensuing thrombosis) relative to current antiplatelet drugs.

Platelet-Dense Granules Worsen Pre-Infection Thrombocytopenia during Gram-Negative Pneumonia-Derived Sepsis

Platelet-dense (δ) granules are important for platelet function. Platelets contribute to host defense and vascular integrity during pneumonia and sepsis, and δ granule products, including adenosine diphosphate (ADP), can influence inflammatory responses. We therefore aimed to study the role of platelet δ granules in the host response during sepsis. Hermansky-Pudlak syndrome (Hps)3coa mice (with reduced δ granule content), mice treated with the platelet ADP receptor inhibitor clopidogrel, and appropriate control mice were infected with the human sepsis pathogen Klebsiella pneumoniae via the airways to induce pneumonia and sepsis. In order to override potential redundancy in platelet functions, we also studied Hps3coa and control mice with moderate antibody-induced thrombocytopenia (10%) prior to infection. We found that sepsis-induced thrombocytopenia tended to be less severe in Hps3coa mice, and was significantly ameliorated in Hps3coa mice with low pre-infection platelet counts. Bacterial growth was similar in Hps3coa and control mice in the presence of normal platelet counts prior to infection, but lower in the lungs of Hps3coa mice with low pre-infection platelet counts. Hps3coa mice had unaltered lung pathology and distant organ injury during pneumosepsis, irrespective of pre-infection platelet counts; lung bleeding did not differ between Hps3coa and control mice. Clopidogrel did not influence any host response parameter. These data suggest that platelet δ granules can play a detrimental role in pneumosepsis by aggravating thrombocytopenia and impairing local antibacterial defense, but that these unfavorable effects only become apparent in the presence of low platelet counts.

Mechanisms of receptor shedding in platelets

The ability to upregulate and downregulate surface-exposed proteins and receptors is a powerful process that allows a cell to instantly respond to its microenvironment. In particular, mobile cells in the bloodstream must rapidly react to conditions where infection or inflammation are detected, and become proadhesive, phagocytic, and/or procoagulant. Platelets are one such blood cell that must rapidly acquire and manage proadhesive and procoagulant properties in order to execute their primary function in hemostasis. The regulation of platelet membrane properties is achieved via several mechanisms, one of which involves the controlled metalloproteolytic release of adhesion receptors and other proteins from the platelet surface. Proteolysis effectively lowers receptor density and reduces the reactivity of platelets, and is a mechanism to control robust platelet activation. Recent research has also established clear links between levels of platelet receptors and platelet lifespan. In this review, we will discuss the current knowledge of metalloproteolytic receptor regulation in the vasculature with emphasis on the platelet receptor system to highlight how receptor density can influence both platelet function and platelet survival.

Nbeal2 Deficiency Increases Organ Damage but Does Not Affect Host Defense During Gram-Negative Pneumonia-Derived Sepsis

Objective- Nbeal2-/- mice, a model of human gray platelet syndrome, have reduced neutrophil granularity and impaired host defense against systemic Staphylococcus aureus infection. We here aimed to study the role of Nbeal2 deficiency in both leukocytes and platelets during gram-negative pneumonia and sepsis. Approach and Results- We studied the role of Nbeal2 in platelets and leukocytes during murine pneumonia and sepsis by Klebsiella pneumoniae. Apart from platelet α-granule deficiency and reduced neutrophil granularity, also monocyte granularity was reduced in Nbeal2-/- mice, whereas plasma levels of MPO (myeloperoxidase), elastase, NGAL (neutrophil gelatinase-associated lipocalin), and MMP-9 (matrix metalloproteinase 9), and leukocyte CD11b expression were increased. Nbeal2-/- leukocytes showed unaltered in vitro antibacterial response and phagocytosis capacity against Klebsiella, and unchanged reactive nitrogen species and cytokine production. Also during Klebsiella pneumonia and sepsis, Nbeal2-/- mice had similar bacterial growth in lung and distant body sites, with enhanced leukocyte migration to the bronchoalveolar space. Despite similar infection-induced inflammation, organ damage was increased in Nbeal2-/- mice, which was also seen during endotoxemia. Platelet-specific Nbeal2 deficiency did not influence leukocyte functions, indicating that Nbeal2 directly modifies leukocytes. Transfusion of Nbeal2-/- but not of Nbeal2+/+ platelets into thrombocytopenic mice was associated with bleeding in the lung but similar host defense, pointing at a role for platelet α-granules in maintaining vascular integrity but not host defense during Klebsiella pneumosepsis. Conclusions- These data show that Nbeal2 deficiency-resulting in gray platelet syndrome-affects platelets, neutrophils, and monocytes, with intact host defense but increased organ damage during gram-negative pneumosepsis.

Start a fire, kill the bug: The role of platelets in inflammation and infection

Platelets are the main players in thrombosis and hemostasis; however they also play important roles during inflammation and infection. Through their surface receptors, platelets can directly interact with pathogens and immune cells. Platelets form complexes with neutrophils to modulate their capacities to produce reactive oxygen species or form neutrophil extracellular traps. Furthermore, they release microbicidal factors and cytokines that kill pathogens and influence the immune response, respectively. Platelets also maintain the vascular integrity during inflammation by a mechanism that is different from classical platelet activation. In this review we summarize the current knowledge about how platelets interact with the innate immune system during inflammation and infection and highlight recent advances in the field.

Spontaneous bleeding in thrombocytopenia: Is it really spontaneous?

Spontaneous bleeding is a clinical hallmark of thrombocytopenia and can take multiple forms including petechiae, epistaxis, gum bleeding, or, in worst cases, intracranial hemorrhage. Those bleeding events are called " spontaneous " because they occur in the absence of overt trauma. Spontaneous bleeding manifestations have long been considered to be a direct consequence of low platelet counts. Nevertheless, although low platelet counts may lead to ultrastructural endothelial alterations, those alterations and the associated state of vascular fragility are unlikely sufficient to cause spontaneous rupture of the microvessel wall. Indeed, in addition to endothelial injury, factors capable of damaging the basement membrane are required to allow escape of red blood cells in the extravascular space. Therefore, despite their misleading name, spontaneous bleeding events in thrombocytopenia are most likely provoked and involve subclinical biological processes in which platelets normally intervene to ensure hemostasis. In this review, we discuss past and more recent studies on the possible triggers of spontaneous bleeding events in thrombocytopenia, with a particular focus on the role of inflammatory reactions.

Role of Peptidylarginine Deiminase 4 in Neutrophil Extracellular Trap Formation and Host Defense during Klebsiella pneumoniae-Induced Pneumonia-Derived Sepsis

Peptidylarginine deiminase 4 (PAD4) catalyzes citrullination of histones, an important step for neutrophil extracellular trap (NET) formation. We aimed to determine the role of PAD4 during pneumonia. Markers of NET formation were measured in lavage fluid from airways of critically ill patients. NET formation and host defense were studied during pneumonia-derived sepsis caused by Klebsiella pneumoniae in PAD4^+/+ and PAD4^-/- mice. Patients with pneumosepsis, compared with those with nonpulmonary disease, showed increased citrullinated histone 3 (CitH3) levels in their airways and a trend toward elevated levels of NET markers cell-free DNA and nucleosomes. During murine pneumosepsis, CitH3 levels were increased in the lungs of PAD4^+/+ but not of PAD4^-/- mice. Combined light and electron microscopy showed NET-like structures surrounding Klebsiella in areas of CitH3 staining in the lung; however, these were also seen in PAD4^-/- mice with absent CitH3 lung staining. Moreover, cell-free DNA and nucleosome levels were mostly similar in both groups. Moreover, Klebsiella and LPS could still induce NETosis in PAD4^-/- neutrophils. Both groups showed largely similar bacterial growth, lung inflammation, and organ injury. In conclusion, these data argue against a major role for PAD4 in NET formation, host defense, or organ injury during pneumonia-derived sepsis.

Glycoprotein VI in securing vascular integrity in inflamed vessels

Glycoprotein VI (GPVI), the main platelet receptor for collagen, has been shown to play a central role in various models of thrombosis, and to be a minor actor of hemostasis at sites of trauma. These observations have made of GPVI a novel target for antithrombotic therapy, as its inhibition would ideally combine efficacy with safety. Nevertheless, recent studies have indicated that GPVI could play an important role in preventing bleeding caused by neutrophils in the inflamed skin and lungs. Remarkably, there is evidence that the GPVI-dependent hemostatic function of platelets at the acute phase of inflammation in these organs does not involve aggregation. From a therapeutic perspective, the vasculoprotective action of GPVI in inflammation suggests that blocking of GPVI might bear some risks of bleeding at sites of neutrophil infiltration. In this review, we summarize recent findings on GPVI functions in inflammation and discuss their possible clinical implications and applications.